1. Field of the Invention
The present invention relates a method of operating an electron tube and, more particularly, to a method of operating a cold-cathode-equipped electron tube in the process of manufacturing the tube.
2. Description of the Prior Art
In the process of manufacturing a cathode ray tube equipped with a field emission cathode, in the "cathode activation step" of extracting electron beams from the cathode for the first time, positive voltages are applied to the first, second, and third focusing electrodes, and the ground potential is applied to the anode (screen and phosphors), thereby raising the voltage at each gate electrode from the ground potential.
At this time, the electron beam emitted from each emitter bombards any one of the first to third focusing electrodes and flows into its power supply. In this state, the temperatures of cold cathode chips such as emitters and gate electrodes and the first to third focusing electrodes rise, and adsorbed gas molecules are emitted owing to electron bombardment on the electrodes. As a result, the degree of vacuum in the tube drops.
As the degree of vacuum in the tube drops as described above, positive ions (cations) are more readily generated due to collisions of gas molecules with electron beams, resulting in an increase in the number of positive ions generated in accordance with the degree of vacuum. The generated positive ions are accelerated toward the cathode and bombard the gate electrodes and emitters of the cathode. As a result, these component may deform. When a positive ion bombards the tip of an emitter, the tip having a radius curvature of about 10 nm may be cut to have a larger radius curvature. This may decrease the electron emission efficiency. Since this change in the shape of the tip of the emitter is an irreversible change, if the impact of positive ion bombardment in the process is strong, the resultant deterioration lasts permanently.
After a lapse of sufficient time since the emission of electrons, adsorbed gas emission is saturated or decreases. At the same time, the degree of vacuum is restored owing to the gettering effect in the tubes. For this reason, the impact of positive ion bombardment can be suppressed below an allowable degree.
Likewise, in the process of manufacturing a traveling wave tube equipped with a field emission cathode, positive ion bombardment may degrade the cathode.